1. Field of the invention
The invention concerns a system devised in order to obtain an automatic compensation of the transversal oscillation in the scanning plane of a laser light profilometer through the application of a device allowing to decrease the time required to assess an unknown profile of any mechanical object or of any detail of the object itself.
2. The Prior Art
As is know, in order to assess the profile of an object it is necessary to measure the values of vertical coordinates (height, diameter, etc.) coming from the points in the profile corresponding to a certain number of values in the horizontal coordinates, obviously measured starting from certain points of origin suitably selected.
In the laser light profilometer as per our patent application registered in Italy with N. 82809 A/82 presented by the applicants of this patent and which represents the current state of the art, the measurement of vertical coordinates is carried out by means of a laser light gauge similar to the one described in the patent granted in Italy with N. 1212675 following application 82817 A/82 (submitted by the applicant). The measurement of the horizontal coordinate is executed by means of a linear optical transducer coupled by a faceplate slide with motor which shifts the piece perpendicularly to the scanning plane of the laser beam.
The values of vertical and horizontal coordinates, read by the profilometer, are memorized and properly processed as input data in an external computer and supply all features of the profile.
Thanks to this profilometer and to this instrumentation we can already obtain a speed, a precision and a flexibility in assessing the profile that are greater than those obtained by means of traditional tecniques; notwithstanding, the aforesaid profilometer does present some limitations attributable to possible imperfections in some optical-mechanical components possibly present in the gauge itself.
In particular, we are referring to the polygonal rotating mirror, whose faces contain some parallelism error vis-a-vis the rotation axis; we also refer to the unavoidable mechanical movement of the bearings supporting the rotation axis of the mirror.
Taking into account that the mirror is, as a rule, joined to a flywheel (since, in fact, the constant rotational speed of the mirror is connected with the precision of the measurement of the vertical coordinate of the profile), the mechanical movement of the bearings accordingly brings about a precession movement of the couple mirror-flywheel around the geometric axis of rotation of the bearings.
The above-mentioned causes (that is, the non-parallelism of the faces and the precession movement) lead to shifts in the scanning plane from one scanning cycle to the next of the laser beam; such displacements can be up to one hundredth of millimeter and involve a similar error in the measurement of horizontal coordinates. On the contrary, since measurements on the vertical axis are extremely precise, all errors on the horizontal axis lead to a deformed piece profile as reconstructed by the computer and, therefore, they spoil the eventual result, especially when the piece presents major variations of gradient, or when the edge of the piece is parallel or almost parallel to the scanning direction.
It is not easy task to limit such error acting directly on the relevant causes, and, besides involving considerable costs, this also spoils the potential of the instrument itself.
Up to now, in order to compensate for the imperfect parallelism of the polygonal mirror faces, only one of its faces was a reflecting face or, alternatively, a single faced mirror was resorted to; this, however, brought about a decreased scanning frequency and, therefore, increasing assessment times.
Another solution envisaged (described in the application for patent registered in Italy with N. 82809 A/83) was to utilize a mirror with faces that could be adjusted separately; this option however had a considerable manufacturing cost. In any case, even though we could imagine to have a perfect mirror at our disposal, the problem of bearing movement would be unsolved all the same.